More and more efforts are endeavored in the ultra-sensitive quantification of peptides and proteins, because many important peptides and proteins are present at ultra-low levels and only the quantity of them or the changes in their abundance really reflects the status and changes of a biological system. We herein report a novel ultra-sensitive method for the simultaneous quantification of three β-casomorphins (β-CMs) based on the Eu3+ diethylenetriamine-N,N,N′,N′′,N′′-pentaacetic acid labeling and capillary electrophoresis with on-line inductively coupled plasma mass spectrometry detection (CE-ICP-MS). The europium-tagged β-CMs were effectively separated by capillary electrophoresis and were sensitively determined by inductively coupled plasma mass spectrometry. Based on the europium-tagging and CE-ICP-MS, three β-CMs were baseline separated and detected within 15 min with an extremely low detection limit of 1.79–2.13 amol (1.79–2.13 × 10−11 mol L−1 for 100 nL of sample injection) and a RSD (n = 4) < 6%. The proposed method has been successfully used to detect β-CMs in real cheese samples with a recovery of 85–103%, suggesting that our method is sensitive and reliable. The success of the present method provides a new possibility for biological assays and clinical diagnoses and a new potential for simultaneous detection of low-abundant proteins or multiple biomarkers.
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